dcx_migration: FileFromUrl doesn't validate files

[IT-Cru]

In transform method of FileFromUrl class in dcx_migration files aren't validated (no hook_file_validate is called) and not created very well (only with uri and filename).

I think this import should be refactored to work more like in DropzoneJsUploadSave class.

[IT-Cru]

Have build a little patch for this on gist
https://gist.github.com/IT-Cru/866dc78a766821121c8559f08456f2ec